KR101720138B1 - Method and tubular semi-finished product for producing an optical fiber - Google Patents

Abstract

A process for producing an optical fiber by stretching a coaxial group of quartz glass blank or quartz glass element based on the obtaining of a fiber comprising a core zone, an inner jacket zone surrounding the core zone, and a ring zone surrounding the inner jacket zone Is known. From this, in order to provide a method, a method of manufacturing a tubular semi-finished product and a group of coaxial elements for the cost-effective production of an optical fiber characterized by a high quality boundary between the core and the jacket and a low bending sensitivity, The silica glass is provided in the form of a ring zone tube made of silica glass having an average fluorine content of at least 6000 wt.ppm, said tube having an inner tube surface and an outer tube surface, A radial fluorine concentration profile with an internal fluorine depletion layer having a layer thickness of at least 1 [mu] m to 10 [mu] m or less is adjusted, the fluorine content decreases toward the inner tube surface, And is not more than 3000 wt.ppm in the adjacent region.

Description

METHOD AND TUBULAR SEMI-FINISHED PRODUCT FOR PRODUCING AN OPTICAL FIBER BACKGROUND OF THE INVENTION [0001]

The present invention relates to a method for manufacturing an optical fiber by stretching a coaxial assembly of quartz glass preform or quartz glass element, said fiber comprising a core zone having a refractive index n _K , And an inner cladding zone having a refractive index n _Mi , a ring zone comprising doped quartz glass surrounding the inner cladding zone and having a refractive index n _F , as well as an outer cladding zone surrounding the ring zone, wherein n _F is a <n _Mi <n _K.

The present invention also relates to a tubular semi-finished product for manufacturing an optical fiber.

The attenuation of the emitted light signal in the optical fiber is changed in particular with the bending of the fiber. A small bending radius (bending radii) affects increased light attenuation. Signal loss can be reduced using bending-insensitive optical fibers. Such fibers have been known for many years and are more concerned with installing fiber-to-the-home (FTTH) to the home. In such applications, particularly small bending radii are often desirable due to space constraints or aesthetic requirements.

Such a bend-reinforced optical fiber is described in WO 2010/003856 A1, and a manufacturing method for the fiber according to the above-described kind is also disclosed. The bend-reinforced optical fiber includes a core zone having a refractive index n _K , a cladding zone having an over-cladding of the core zone and having a refractive index n _Mi , a ring zone formed of a fluorine-doped quartz glass having a refractive index n _F surrounding the cladding zone as a ring surrounding the said zone comprises an outer layer composed of quartz glass that is not doped with a refractive index n and _Ma, causing the radial refractive index profile, wherein _{n Ma> n F <n Mi} <n K. The fibers obtained becomes, tubular or rod by stretching of the coaxial assembly of the quartz glass mother material or the quartz glass component-ring zone layer on the substrate material of the shape is fluorine having a layer thickness and refractive index n _F ≤ 1.4519 of at least 1 ㎜-doped The quartz glass in the ring zone is produced by a plasma external deposition process.

In order to obtain an efficient manufacturing method and to minimize the risk of waste, it is advantageous if each element of the fiber is manufactured in a separate process step and is only bonded at the final step.

Such a process is described in EP 1 712 934 A1, which also discloses semi-finished products according to the kind mentioned above. In order to produce a base material having a large volume, only semi-finished products consisting of a plurality of tubular or rod-shaped individual elements are provided. The individual elements consist of a core rod formed by combining end and end stacked segments and arranged in an inner overclad tube. The inner overclad tube is surrounded by an outer overclad tube. The elements are arranged coaxially with one another and are drawn directly to the optical fiber in the drawing process.

Metal impurities and hydroxyl groups such as chlorine, aluminum or iron can negatively affect the light transmission of the drawn fiber. To minimize their effects, the internal overclad tube exhibits high purity, expensive quartz glass quality. On the other hand, in order to keep the cost of the semi-finished product low, it has been proposed that the outer overclad tube should have a higher impurity concentration.

It is true that the separation of the individual elements and the final combination in the fiber drawing process are beneficial for the inexpensive fabrication of optical fibers and the reduction of waste risk. This, however, automatically causes additional surfaces and interfaces between the individual elements to be combined, increases interface problems, and increases the need for interface quality. Particular attention should be paid to the formation of bubbles in the interface region.

DE 10 2008 047 736 B3 describes the use of a fluorine-doped quartz glass tube to overcladd the core rod.

US 6,263,706 B1 describes a method for establishing a fluorine-concentration profile in a SiO ₂ soot tube. Since the fluorine-containing doping gas is passed along the outer wall of the soot tube, and the gas and helium do not pass through the inner bore, the soot tube is preferably fluorine-doped through the gas phase after removal of the mandrel. The resulting refractive index profile has a maximum value at the tube center and is substantially parabolic.

DE 600 04 778 T2 describes the preparation of fluorine-doped quartz glass bodies via sol-gel-route. Fluorine doping is performed by adding a sol, preferably tetramethylammonium fluoride, to the fluorine-containing initial substrate. After sintering the cylindrical quartz glass, the sample shows a refractive index curve when the refractive index decreases from outside to inside (and thus the fluorine concentration increases from outside to inside). The highest fluorine concentration is obtained at the center. The decrease in fluorine concentration at the edge is due to fluorine loss due to heating.

US 3,981,707 A discloses a method of disrupting a fluorine-doped quartz glass tube having a non-uniform radial fluorine concentration profile when the fluorine concentration has a maximum at the tube center and decreases toward the inner tube wall and the outer tube wall, &Lt; / RTI &gt; Since the tube composed of fluorine doped quartz glass is treated at high temperature, fluorine depletion is formed in the region without the tube surface.

DE 101 55 134 C1 describes a method for producing a base material, wherein the core rod is provided as a soot layer by a POD (Plasma External Deposition) process in a hydrogen-free atmosphere at a predetermined cladding / core ratio, Is then dried and vitrified. A hydroxyl group content of less than 30 wt. Ppb and a zero-contact surface on the core rod are obtained in the cladding glass.

It is an object of the present invention to provide a method and a tubular semi-finished product for inexpensively manufacturing an optical fiber characterized by a high-quality interface between the core and the cladding and a low bending sensitivity.

With respect to the production process, this object, starting from the above-mentioned method, is provided in that the quartz glass of the ring zone is in the form of a ring zone tube made of quartz glass having an average fluorine content of at least 6000 wt.ppm, Wherein the ring zone tube has a wall formed of an inner tube surface and an outer tube surface, wherein a radial fluorine concentration profile is set on the wall of the ring zone tube, the profile having an inner surface having a layer thickness of at least 1 [mu] Fluorine depletion layer, the fluorine content being obtained according to the present invention wherein the fluorine content is reduced towards the inner tube surface and is no more than 3000 wt.ppm in the region near the surface having a thickness of 1 mu m.

According to the present invention, the quartz glass for the ring zone is provided in the form of at least one fluorine doped quartz glass tube (referred to herein as a " ring zone tube ").

SiO ₂ particles are directly fluorine doped silica glass tube so-called plasma outside deposition are directly vitrified as (POD; p lasma o utside d eposition) of the SiO ₂ soot body is doped with the aid of the process, or after the fluorine and sintered A fluorine doped quartz glass tube is produced with the aid of the CVD method by hydrolysis or pyrolysis of the silicon-containing starting compound by build-up.

The outer cladding tube of the doped or undoped quartz glass and the inner cladding zone of the fiber and the core rod for producing the core zone for producing the other individual elements, i.e. the outer cladding zone of the fiber, And may then be arranged in the form of an assembly of coaxial elements for producing fibers. The coaxial assembly is then collapsed into the base material, stretched, or drawn directly into the fiber. If the parent material is fabricated first, the parent material is drawn into the optical fiber in a separate method step. In this process, or before that, it may be provided as an additional cladding material, for example by adding additional quartz glass in the form of a tube or by an external deposition method.

In single mode fibers, the ring zone of fluorine doped quartz glass with a lower refractive index than cladding glass affects additional conduction of the light mode. In order to prevent excessive deformation in the mode field diameter compared to standard monomode fibers, the ring zone has a slight distance from the core, i.e. the ring zone is defined by the inner cladding glass zone And is separated from the core. Thus, ring zones affect loss reduction during fiber bending compared to standard fibers that do not compromise compatibility with standard single mode fibers. This is important when the bending reinforcing fibers having ring zones are connected to standard fibers.

In the case where the contact surface is formed toward the core zone, it is important that a fluorine-doped ring zone tube is provided on the wall with a decreasing radial fluorine concentration profile toward the inner tube surface. This has been found to substantially reduce the formation of interfacial bubbles including the core rod in the stretching process. The region where the fluorine concentration is reduced at the edge portion of the ring zone tube is hereinafter referred to as a "fluorine depletion layer &quot;.

Doping with fluorine affects the reduction of the refractive index of the quartz glass of the ring zone. For high bending enhancement of the optical fiber, a high fluorine concentration and a steep refractive index jump between the core rod and the ring zone are advantageous. Therefore, the fluorine content is set to at least 6000 wt. Ppm and may be an amount of up to 15,000 wt. Ppm. The reduction in refractive index achieved by doping fluorine compared to undoped quartz glass is at least 2 x 10 ^-4 relative to the refractive index of undoped quartz glass.

However, on the other hand, problems associated with interfacial bubbles increase within the increasing average fluorine content of the ring zone tube. For this reason, the bubble-reducing effect of the fluorine depletion layer is particularly pronounced in the case of high average fluorine content. Thus, the process according to the invention is particularly pronounced in ring zone tubes having a high average fluorine content of 6000 wt.ppm or more. The fluorine concentration according to the spectroscopic method across the tube wall is understood as the average fluorine content of the ring zone tube. The fluorine depletion layer is not significantly noticeable due to its relatively thin thickness.

Doping with fluorine affects the reduction of the refractive index of the quartz glass of the ring zone. As described above, the steep refractive index jump between the core rod and the ring zone is advantageous for high bending strength of the optical fiber. Fluorine concentration gradients and the resulting radial refractive gradient are in themselves not preferred at this point. Surprisingly, however, it has been found that a layer thickness of less than 10 [mu] m is permissible and exhibits a good compromise between fiber bend enhancement and interface quality. However, at layer thicknesses of less than 1 [mu] m, there is no significant effect on reduced bubble formation.

The lower the fluorine content in the fluorine depletion layer, the less the problem of bubble formation on the contact surface of the ring zone tube. In this regard, the fluorine depletion layer in a region near the surface having a thickness of 1 占 퐉 has a fluorine content of 3000 wt ppm or less. A particularly low fluorine content in the region near the surface contributes substantially to the formation of low bubbles.

The fluorine depletion layer is formed with a layer thickness having a fluorine concentration that is less than 80% of the maximum concentration of fluorine in the quartz glass of the ring zone tube. Likewise, the region near the fluorine depletion surface is formed with a layer thickness having a fluorine concentration that is less than 80% of the maximum concentration of fluorine in the quartz glass of the ring zone tube.

What is crucial is a reduction in the fluorine concentration profile towards the core rod. It has been found, however, that it is advantageous if a radial fluorine concentration profile is provided in the ring zone tube on the wall of the ring zone tube which also includes an external fluorine depletion layer in which the fluorine concentration decreases towards the outer tube surface.

This results in reduced bubble formation at the interface between the ring zone and the outer cladding zone. Less strictly for the dimensions of the outer fluorine depletion layer, a steep decrease in the fluorine concentration, and a maximum concentration in the region near the surface than in the case of the inner fluorine depletion layer. The dimensions, profile and concentration already described above for the internal fluorine depletion layer are suitable in any ratio to the external fluorine depletion layer.

It has been found advantageous when the fluorine depletion layer has a layer thickness of less than 4 [mu] m.

A layer thickness of less than 4 [mu] m is a particularly good compromise between the bending enhancement of the fiber and the internal interface quality of the ring zone tube.

Preferably, the ring zone tube is manufactured without tools by drawing the base tube from the fluorine-doped quartz glass while forming a fluorine depletion zone on the inner tube surface and the outer tube surface.

However, heating of the base tube and accompanying out-diffusion of fluorine in the layer near the surface generally causes fluorine depletion on the inner and outer surfaces, resulting in a flat temperature gradient. The fluorine outer-diffusion can be set by selecting the temperature and by continuing to heat during the stretching of the base tube, so that the fluorine depletion layer is obtained with the above-mentioned dimensional and concentration profile. A suitable pair of parameters of sustain temperature / heating can be determined by several tests of the supplier.

The additional indicator is the outer diameter of the drawn ring zone tube, preferably in the range of 10-50% of the outer diameter of the base tube; Generally, an outer diameter in the range of 30-70 mm is obtained, and the cylinder ratio of the outer diameter to the inner diameter is 0.2-0.8 less in the ring zone tube than in the base tube.

Alternatively, the ring zone tube is heated after being manufactured in a separate method step for producing a fluorine depletion layer having a desired thickness and a predetermined fluorine concentration profile. Also, in the case where the ring zone tube is heated, the fluorine out-diffusion can be set by selecting temperature and continuous heating during some tests.

Heating the base tube in the drawing process or heating the ring zone tube is preferably carried out with an electric heater in a hydrogen-free atmosphere.

This reduces the injection of hydroxyl groups into the region near the ring zone tube surface.

Without any special precautions, a layer of fluorine depletion surface having a thickness exceeding the target size can be obtained after drawing of the base tube. The result can also be obtained during heating of the ring zone tube for the purpose of setting a sufficiently thick fluorine depletion layer. The excess fluorine depletion layer thickness may be removed by etching with hydrofluoric acid or a gaseous etchant. Therefore, it has proved useful to partially remove at least the inner tube surface of the ring zone tube after or after drawing of the base tube.

Because of the removal of the inner surface, the thickness of the fluorine depletion layer decreases to a predetermined desired value, and at the same time a clean surface free of possible defects is produced. Preferably, the removal process is performed in a vapor phase etching manner to avoid loading of quartz glass containing water and hydroxyl groups.

The thickness of the fluorine depletion surface layer can be 15 [mu] m or more after drawing of the base tube or after heating of the ring zone tube. In subsequent removal of the inner surface, it is important that the removal depth is less than the previously fabricated fluorine depletion surface layer, and a portion thereof is maintained as a fluorine depletion layer with the desired target thickness. In this regard, it has been found advantageous when the removal thickness of the inner surface is in the range of 10-70% of the original thickness of the fluorine depletion surface layer.

The lower the fluorine content in the fluorine depletion layer, the less is the problem with bubble formation on the contact surface of the ring zone tube. In this regard, it has been found advantageous if the fluorine depletion layer in the region near the surface has a fluorine content of 2000 wt ppm or less. A low fluorine content in the near-surface layer having a layer thickness of 1 占 퐉 substantially reduces bubble formation.

It has been found that a high fluorine content and a combined hydroxyl group content can promote bubbles in the region of the contact surface of the ring zone tube. Thus, a procedure is preferred in which the quartz glass of the ring zone tube in the region of the fluorine depletion layer has an average hydroxyl group content of less than 1 wt.ppm, preferably less than 0.5 wt.ppm.

The lower hydroxyl group content in the fluorine depletion layer can be obtained, for example, by heating during the drawing of the base tube or by heating the ring zone tube to form a fluorine depletion layer to obtain a ring zone tube, Lt; / RTI &gt;

The low hydroxyl group content is equally useful for other discrete elements that are bonded to the ring zone tube to avoid bubble formation. For this reason, the quartz glass of the inner cladding zone has a thickness of 0.5 wt. Ppm in the region of the surface layer having a layer thickness of 10 mu m. Lt; RTI ID = 0.0 &gt; of less &lt; / RTI &gt; hydroxyl groups.

The low hydroxyl group content in the region near the surface of the core rod is obtained, for example, by stretching the core rod in the electric heating zone, that is, the hydrogen containing flame is not used for the flame polishing or the like.

In addition, the quartz glass of the outer cladding zone has a surface area of 0.5 wt. Ppm. In the inner surface area of the surface layer having a layer thickness of 10 mu m. Lt; RTI ID = 0.0 &gt;%&lt; / RTI &gt; hydroxyl group content.

The outer cladding tube is preferably made of doped or undoped quartz glass and is preferably made by drawing into a sintering and cladding tube comprising an OVD soot-buildup process and subsequent dewatering in an atmosphere containing chlorine, and a mechanical treatment .

Also, the low hydroxyl group content in the inner surface area of the outer cladding tube is realized in that the inner surface of the outer cladding tube is made by mechanical treatment.

Due to the mechanical treatment, the possibly increased hydroxyl enriched content in the surface area can be removed without the introduction of new hydroxyl groups.

In addition, the heating of the ring zone tube to produce the fluorine depletion layer can be performed during the stretching of the assembly consisting of carbon, ring zone tube and possibly additional individual parts. The length of the heating zone and the feed rate of the assembly through the heating zone. In this respect, it has been found advantageous when the heating zone length (mm) and the feed rate of the base tube (mm / min) are 10 min or more.

The length of the heating zone is at least 150 mm, generally in the range of 180 mm to 250 mm. The temperature within the surface area of the ring zone tube is at least 2200 ° C.

With respect to the tubular semi-finished product for manufacturing an optical fiber, the above-mentioned object is achieved when the tubular semi-finished product is made of quartz glass having an average fluorine content of at least 6000 wt.ppm and has a wall formed by an inner tube surface and an outer tube surface, Wherein the profile has an internal fluorine depletion layer having a layer thickness of at least 1 [mu] m to 10 [mu] m and the fluorine content decreases toward the inner tube surface and has a thickness of 1 [mu] m In the region near the surface, 3000 wt. ppm or less, according to the present invention.

Semifinished products in the form of fluorine doped quartz glass tubes are referred to herein as "ring zone tubes &quot;. The tubes are SiO ₂ particles are directly Fluorine is the so-called plasma outside deposition synthetically directly vitrified quartz glass pipe doped (POD; p lasma o utside d eposition) with the aid of the method, or is doped with fluorine after the With the aid of the build-up of a sintered SiO ₂ soot body and the CVD method by hydrolysis or pyrolysis of the silicon-containing starting compounds.

The ring zone tube is provided for manufacturing the optical fiber in the above-described manner, and is made of a base material or fiber in the coaxial assembly together with other separate elements, including an outer cladding tube and a core lobe. The inner tube surface and the outer tube surface are softened and combined with the quartz glass of the other discrete elements constituting the quartz glass. Due to the fluorine content of the ring zone tube, bubbles can be formed on the contact surface.

To cope with this effect, the ring zone tube according to the invention is provided with a decreasing radial fluorine concentration profile on its walls at least towards the inner surface. As a result, bubble formation on the interface of the core rod is substantially reduced during the stretching of the semi-finished product. The portion where the fluorine concentration is reduced at the edge portion of the ring zone tube is called a "fluorine depletion layer &quot;.

Doping with fluorine affects the reduction of the refractive index of the quartz glass of the ring zone. For high bending enhancement of the optical fiber, a high fluorine concentration and a steep refractive index jump between the core rod and the ring zone are advantageous. Therefore, the fluorine content of the ring zone tube is at least 6000 wt.ppm and can be up to 15,000 wt.ppm. The reduction in refractive index caused by fluorine doping compared to undoped quartz glass is at least 2 x 10 ^-4 relative to the refractive index of undoped quartz glass.

However, on the other hand, the problem with interfacial bubbles increases within the increased average fluorine content of the ring zone tube. For this reason, the bubble reduction effect of the fluorine depletion layer is particularly remarkable in the case of high average fluorine content. Thus, the process according to the invention is particularly pronounced in ring zone tubes having a high average fluorine content of 6000 wt.ppm or higher.

Doping with fluorine affects the reduction of the refractive index of quartz glass in the ring zone. As described above, the steep refractive index jump between the core rod and the ring zone is advantageous for high bending strength of the optical fiber. Fluorine concentration gradients and the resulting radial refractive gradient are in themselves not preferred at this point. However, a layer thickness of less than 10 [mu] m is permissible and represents a suitable compromise between the bending enhancement of the fibers and the quality of the interface. However, there is no significant effect on reduced bubble formation at layer thicknesses of less than 1 [mu] m.

The lower the fluorine content in the fluorine depletion layer, the less is the problem with the formation of bubbles on the contact surface of the ring zone tube. In this regard, the fluorine depletion layer in a region near the surface having a thickness of 1 占 퐉 has a fluorine content of 3000 wt.ppm or less. A particularly low fluorine content in the region near the surface contributes substantially to low bubble formation.

The region near the fluorine depletion layer and the fluorine depleted surface was further confirmed above with reference to the method according to the present invention.

Advantageous developments of the semi-finished products according to the invention are in accordance with the claims. In the arrangement of semi-finished products shown in the claims section, the procedure is set forth in the claims for the method according to the invention, and the statement for the corresponding method claim is referred to for further explanation.

The ring zone tube is provided to fabricate the optical fiber. For this purpose, the ring zone tube is arranged with other individual elements, including the outer cladding tube and the core rod of doped or undoped quartz glass, which are each produced in a separate process in the form of a component assembly . The assembly is then fabricated either as a base material or as a direct fiber. It is important that the fluorine doped ring zone tube has a decreasing radial fluorine concentration profile across its walls to the inner surface. As a result, bubble formation at the interface of the core rod during stretching of the semi-finished product is considerably reduced.

The ring zone tube results in increased bending enhancement in the drawn fiber.

Preferably, the core rod is comprised of a plurality of core rod segments arranged end-to-end in the axial direction within the ring zone tube. The division of the core rod allows a core rod length of any desired dimension.

An annular gap, preferably with a gap width in the range of 0.5 to 2 mm, lies in the assembly between the core rod and the ring zone tube. The same is true for the ring gap between the ring zone tube and the outer cladding tube.

The ratio of the outer diameter to the inner diameter of the ring zone tube is preferably in the range of 1.4 to 1.8 in a wall thickness of 2 to 12 mm. The length of the ring zone tube is generally 1 to 3 m, and the plurality of ring zone tube sections may be joined to each other at their ends.

The present invention will be described in more detail with reference to patent drawings and embodiments.
Figure 1 is a diagram of the radial concentration profile of SiO ₂ and fluorine in the outer tube surface area of the ring zone tube.
Figure 2 is a diagram of the radial concentration profile of SiO ₂ and fluorine in the inner tube surface area of the ring zone tube.

The soot bodies are made with the aid of standard soap deposition methods, and are then doped with fluorine.

After vitrification of the soot body, the quartz glass cylinder is obtained with an outer diameter of 200 mm and an inner diameter of 80 mm, resulting in a ratio of outer diameter to inner diameter of 2.5. The quartz glass has an average hydroxyl group content of 0.1 wt.ppm and an average fluorine content of 6100 wt.ppm, which results in a reduction in refractive index compared to undoped quartz glass.

The initial cylinder is drawn out toolless with a ring zone tube having an outer diameter of 38 mm, an inner diameter of 25 mm and a wall thickness of 6.5 mm.

The initial cylinder was charged with hydrogen in an electric heating zone at a rate of 170 mm in length " L "at a feed rate" v "of 10 mm / min (resulting in an exponent L / v of 10 min) In a nitrogen atmosphere.

The ring zone tube obtained later is divided into inner tube surfaces softened by hot forming, and has a particularly high quality surface. A fluorine depletion surface layer having a thickness of about 5 占 퐉 is formed by heating during the stretching process in the region of the inner and outer tube surfaces of the ring zone tube.

The corresponding concentration profile in the area of the outer surface is shown in Fig. The concentrations for fluorine (Curve 2) and SiO ₂ (Curve 1) are plotted on the ordinate in relative units (based on the respective maximum concentrations of SiO ₂ and fluorine) and the radial position is plotted on the abscissa [ ]. The gradual rise in each concentration profile and the offset from zero are due to the spatial resolution of the measured method.

The layer thickness having a fluorine concentration of less than 80% of the maximum value is referred to as a "fluorine depletion layer &quot;. The relevant positions are shown in "B" The concentration of SiO ₂ represents the reference point (zero point) for the position value. The zero point for the position where the zero point is associated with the 80% concentration value is shown in "A " Accordingly, the fluorine layer thickness for the depletion layer follows the distance AB of the fluorine and the concentration profile of the SiO ₂ eseo 80% concentration. In the region near the surface of up to 1 占 퐉, the fluorine concentration is less than 2100 wt.ppm, and the thickness of the fluorine depletion surface layer is generally about 7 占 퐉 in this case. This is allowed on the outer cylinder jacket of the ring zone tube, but not on the inner cylinder jacket.

Thus, after the elongation has a fluorine depletion layer similar to the outer cladding, the inner surface of the ring zone tube is etched off and the hot gas stream of the etching gas (SF ₆ ) passes through the inner bore. The concentration profile obtained on the inner wall is shown in Fig. According to a comparison of the concentration profile of SiO ₂ (curve 2) and fluorine (curve 1), due to the removal of the inner surface, the thickness of the fluorine depletion layer is adjusted to a predetermined desired value of about 1.5 μm, Curves are obtained in the area near the surface. Due to the etching process, a clean, defect-free surface is produced. In this case, the layer thickness, defined as the fluorine depletion layer, has a fluorine concentration that is less than 80% of the maximum concentration of fluorine and can be read in position AB. The average fluorine content in the fluorine depletion layer was 3000 wt. ppm, and is about 2800 wt.ppm in the region near the inner surface of 1 mu m.

It is true that the average chlorine content of the ring zone tube is 200 wt.ppm and the general hydroxyl group content of the quartz glass of the ring zone tube is 0.1 wt.ppm. However, due to the stretching process, it increases to a maximum value of 5 wt.ppm or more near the surface. However, due to the subsequent vapor phase etching, the layer having a relatively high hydroxyl group content is removed so that an average hydroxyl group of 0.4 wt ppm or less is obtained on the surface of the fluorine depletion layer measured at a layer thickness of 1 mu m .

The ring zone tube thus obtained is used for overcladding the core rod by a rod-in-tube method. For this reason, the segment is cut to a desired length from the ring zone tube. The core rod is surrounded by an inner cladding of undoped quartz glass having a GeO ₂ doped core area at a radius of 12 mm and a layer thickness of 5.5 mm.

The core rod was inserted alternately into the inner bore and the tube of the ring zone tube, and an undoped quartz glass with an outer diameter of 175 mm, an inner diameter of 40 mm and an average chlorine content of 1800 wt.ppm and a refractive index of n _Ma It is surrounded by a jacket tube.

This coaxial arrangement of the elements is then injected into a vertically oriented drawing furnace, beginning at the lower end, softened by zone, and the fibers are drawn into the softened region. The outer and inner "fluorine depletion layers" of the ring zone tubes serve as "passivation layers" that reduce the external diffusion of fluorine, thereby preventing the formation of bubbles in the interface region . Thus, the fluorine depletion layer contributes to the interface and low-defect interface of the core rod and jacket tube.

The bend reinforced single mode optical fiber having an outer diameter of 125 占 퐉 is drawn; Characterized by a ring zone having a high fluorine concentration and having a distance from the outer region of the core zone. The flow of the radius of the refractive index of the assembly is as follows: n _Ma > n _F <n _Mi <n _K.

Claims (16)

CLAIMS 1. A method for manufacturing an optical fiber by stretching a coaxial assembly of quartz glass base material or quartz glass element,
The fiber comprises a core zone having a refractive index n _K , an inner cladding zone having a refractive index n _Mi overcladding the core zone, a ring zone consisting of a doped quartz glass having a refractive index n _F surrounding the inner cladding zone, And an outer cladding zone surrounding the ring zone, where n _F &lt; _nMi &lt; _nK ,
Wherein the quartz glass of the ring zone is provided in the form of a ring zone comprised of quartz glass having an average fluorine content of at least 6000 wt.ppm and wherein the ring zone tube has a wall formed of an inner tube surface and an outer tube surface,
A radial fluorine concentration profile is set on the wall of the ring zone tube and the profile has an inner fluorine depletion layer having a layer thickness of 1 占 퐉 to 10 占 퐉,
Wherein the fluorine content decreases toward the inner tube surface and is not more than 3000 wt.ppm in the region near the surface having a thickness of 1 mu m.
The method according to claim 1,
Wherein the ring zone tube is provided with a radial fluorine concentration profile on a wall of the ring zone tube, the profile having an outer fluorine depletion layer in which the fluorine concentration decreases towards the outer tube surface.
The method according to claim 1,
Wherein the inner fluorine depletion layer has a layer thickness of less than 4 [mu] m.
The method according to claim 1,
Wherein the ring zone tube is manufactured without tools by drawing the base tube from the fluorine doped quartz glass while forming a fluorine depletion layer on the inner and outer surfaces.
The method of claim 3,
Characterized in that at least the inner tube surface of the ring zone tube is partially removed after drawing of the base tube or during drawing and the removal depth of the inner tube surface is in the range of 10% to 70% of the original thickness of the fluorine- Gt;
6. The method according to any one of claims 1 to 5,
Wherein the fluorine depletion layer in the near-surface layer having a layer thickness of 1 占 퐉 exhibits a fluorine concentration of 2000 wt ppm or less.
6. The method according to any one of claims 1 to 5,
Wherein the quartz glass of the ring zone tube in the region of the fluorine depletion layer has an average hydroxyl group content of less than 1 wt.ppm.
6. The method according to any one of claims 1 to 5,
Wherein the quartz glass of the ring zone tube in the region of the fluorine depletion layer has an average hydroxyl group content of less than 0.5 wt.ppm.
6. The method according to any one of claims 1 to 5,
The quartz glass of the inner cladding zone is provided by a core rod having an outer surface layer having a layer thickness of 10 mu m and said outer surface layer is a quartz glass having a hydroxyl group content of less than 0.5 wt.ppm Gt;
In a tubular semi-finished product for manufacturing an optical fiber,
The tubular semi-finished product is made of quartz glass having an average fluorine content of at least 6000 wt.ppm and has a wall formed by an inner tube surface and an outer tube surface,
A radial fluorine concentration profile is set on the wall, the profile having an inner fluorine depletion layer having a layer thickness of 1 占 퐉 to 10 占 퐉,
Wherein the fluorine concentration is reduced towards the inner tube surface and less than or equal to 3000 wt.ppm in a near-surface region having a thickness of 1 mu m.
11. The method of claim 10,
Wherein a radial fluorine concentration profile with an outer fluorine depletion layer in which the fluorine concentration decreases towards the outer tube surface is set on the wall of the ring zone tube.
The method according to claim 10 or 11,
Wherein the inner fluorine depletion layer has a layer thickness of less than 4 占 퐉.
The method according to claim 10 or 11,
Wherein the ring zone tube has an inner tube surface manufactured without tools in the melt flow.
The method according to claim 10 or 11,
Wherein the fluorine depletion layer has a fluorine content of less than or equal to 2000 wt.ppm in the region near the surface having a layer thickness of 1 mu m.
The method according to claim 10 or 11,
The quartz glass of the ring zone tube in the region of the fluorine depletion layer contained 1 wt. lt; RTI ID = 0.0 &gt; ppm. &lt; / RTI &gt;
The method according to claim 10 or 11,
Wherein the quartz glass of the ring zone tube in the region of the fluorine depletion layer has an average hydroxyl group content of less than 0.5 wt.ppm.

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